This is a general question I have been puzzling over. We learn that according to Torah law the Kesuva is on land, (though later the
geonim made movable property also usable). But according to Torah law, what if a man has no land? Could he not get married?
Tosfos at the beginning of the fifth Perek asks how can a person guarantee a Kesuvah even if he is penniless. Tosefos offers two possibilities, which are further expanded by Rosh there. The basic idea is that a person can obligate himself to a future debt even if he cannot pay it at present.
As for not having land, the majority opinion of the Rishonim (Kidushin 65b) is that the specification of land is only when the Kesuvah is being paid out of the estate after he dies, when one divorces his wife he is liable to pay from all his assets.